fix: set shell to bash in Makefile to enable unset (#154)

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## Which problem is this PR solving?

- Closes #69

## Short description of the changes

sets shell to bash to fix shell builtin behaviour

## How to verify that this has the expected result

run `make test` with a disruptive environment variable that starts with
`HONEYCOMB_` or `OTEL_` set

Author: to-json

Makefile

@@ -1,3 +1,6 @@
+#: set shell to bash to make shell builtin behaviour consistent
+SHELL := /usr/bin/env bash
+
 #: install dependencies; set dev_only for only development dependencies or use the install_dev target
 install:
 ifeq ($(dev_only),) # dev_only not set